Introduction

Benzimidazole is a heterocyclic aromatic organic compound. It is an important pharmacophore and a privileged structure in medicinal chemistry. This compound is bicyclic in nature which consists of the fusion of benzene and imidazole rings. At present, it is a moiety of choice which possesses many pharmacological properties. The most prominent benzimidazole compound in nature is N-ribosyl-dimethylbenzimidazole, which serves as an axial ligand for cobalt in vitamin B12 (Barker et al., 1960; Walia et al., 2011).

Piperazines are another class of organic heterocyclic compounds that consists of a six-membered ring containing two nitrogen atoms. The piperazines are a broad class of chemical compounds, many with important pharmacological properties, which contain a core piperazine functional group (Merck Index, 1989; Ashford’s Dictionary of Industrial Chemicals, 2011; Chisholm, 1911).

Dopamine is a predominant catecholamine neurotransmitter in the brain, where it controls a variety of functions including locomotor activity, cognition, emotion, positive reinforcement, food intake and endocrine regulation. This catecholamine also plays multiple roles in the periphery as a modulator of cardiovascular function, catecholamine release, hormone secretion vascular tone, renal function, and gastrointestinal motility. Dopaminergic systems have been the focus of research since past 30 years mainly because several pathological conditions, such as Parkinson’s disease, schizophrenia, Tourette syndrome, hyperprolactinemia, substance addiction, affective disorders, and attention-deficit hyperactivity disorders, have been linked to dysregulation of dopaminergic transmission.

Dopamine receptor antagonists have been developed to block hallucinations and delusions that occur in schizophrenia patients, whereas dopamine receptor agonists are effective in alleviating the hypokinesia of Parkinson’s disease. However, desensitization of DA receptors can induce extrapyramidal effects resulting from DA depletion, and high doses of agonists can cause psychoses. The therapies of disorder resulting from dopamine imbalances are thus associated with several side effects. The complementary DNAs of the five distinct DA receptor subtypes (D1–D5) have been infact isolated and characterized by application of gene-cloning procedure. A classical requirement to elucidate the functional role of individual receptor subtypes is identification of selective agonists and antagonists (Missale et al., 1998).

Wide range of antipsychotic drugs have been synthesized until today which have shown their varying affinity toward D1, D2, D3, D4, 5-HT1A, and 5-HT2A receptors. Drugs may or may not have affinity toward all the above receptors, but they have higher selectivity toward a specific receptor where they act as agonist or antagonist. This in turn depends on the nature of the parent molecule and the type of substituent attached to the parent moiety. Dopamine receptor antagonism plays a key role in mediation of antipsychotic effect. Novel antipsychotic derivatives have been synthesized to act at a specific receptor and exert their action against various indications of psychoses, i.e., schizophrenia, persistent delusional disorders, chronic hallucinatory psychoses, bipolar disorder, personality disorder, schizoaffective disorders, and Schizotypy disorders (Kebabian and Calne, 1979; Missale et al., 1998). The review will start with a general study on structure of dopamine receptors. The detailed account of the structures of various benzimidazole piperazinyl compounds and their interaction with enzymes has been summarized. Finally, the synthetic scheme of each compound along with chemistry has been discussed.

Structure of dopamine receptors

Analysis of the primary structure of the cloned DA receptors revealed that they are members of the seven transmembrane (TM) domain G protein-coupled receptor family and share most of their structural characteristics (Fig. 1). Members of this family display considerable amino acid sequence conservation within TM domains (Probst et al., 1992).

Fig. 1
figure 1

Representation of a general dopamine receptor structure (D1-like). The active phosphorylation sites are represented on the –COOH terminal and the third intracellular loop (I3) whereas the active glycosylation sites are represented on –NH2 terminal (Strange 1996). E1–E3:extracellular loops, I2–I3:intracellular loops, 1–7:transmembrane domains

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Analysis of DA receptor structure pointed to similarities and dissimilarities between D1-like and D2-like receptors (Civelli et al., 1993; Gingrich and Caron, 1993; Jackson and Westlind-Danielsson, 1994; O’Dowd, 1993). Members of the same family share considerable homology. The D1 and D5 receptors share an 80 % identity in their TM domains. The D2 and D3 receptors have a 75 % identity in their TM domains, and the D2 and D4 receptors share a 53 % identity in the TM domains. Fig. 1 shows structural features of the D1-like receptors, whereas the D2-like receptors are characterized by a shorter COOH-terminal tail and by a bigger third intracellular loop (Missale et al., 1998).

The NH2-terminal stretch has a similar number of amino acids in all the receptor subtypes and carries a variable number of consensus N-glycosylation sites. The D1 and D5 receptors possess two such sites, one in the NH2 terminal and the other one in the second extracellular loop. The D2 receptor has four potential glycosylation sites, the D3 has three, and the D4 possesses only one (Civelli et al., 1993; Gingrich and Caron, 1993; Jackson and Westlind-Danielsson, 1994; O’Dowd, 1993).

The COOH terminal is about seven times longer for D1-like receptors than for the D2-like receptors (Fig. 1). Likewise, as in all G protein-coupled receptors, DA receptors possess two cysteine residues in extracellular loops 2 and 3 (Civelli et al., 1993; Gingrich and Caron, 1993; Jackson and Westlind-Danielsson, 1994; O’Dowd, 1993), which have been suggested to form an intramolecular disulfide bridge to stabilize the receptor structure (Dohlman et al., 1990; Fraser, 1989). The D2-like receptors have a long third intracellular loop, a feature which is common to receptors interacting with inhibitory, Gi proteins to inhibit AC, whereas the D1-like receptors are characterized by a short third loop as in many receptors coupled to stimulatory, Gs protein (Civelli et al., 1993; Gingrich and Caron, 1993; O’Dowd, 1993).

The D1 and D5 receptor’s third intracellular loop and their –COOH terminus are similar in size but divergent in their sequence. In contrast, the small cytoplasmic loops 1 and 2 are highly conserved so that any difference in the biology of these receptors can be probably related to the third cytoplasmic loop and the –COOH terminal tail (Civelli et al., 1993; Gingrich and Caron, 1993; O’Dowd, 1993). The external loop between TM4 and TM5 is considerably different in the two receptor subtypes, being shorter (27 amino acids) in the D1 receptor than in the D5 receptor (41 amino acids). The amino acid sequence of this loop, in addition, is divergent in the D5 and in its rat counterpart D1b (Sunahara et al., 1991; Tiberi et al., 2004).

Site-directed mutagenesis for catecholamine receptors (Kjelsberg et al., 1992; Strader et al., 1989; Strader et al., 1988) and protein modeling with the β2-, α2-, and D2 receptors (Hibert et al., 1992; Hibert et al., 1993; Trumpp-Kallmeyer et al., 1992) suggested that the agonist binding likely occurs within the hydrophobic TM domains (Fig. 1). Highly conserved residues are present in the core of the protein and define a narrow binding pocket that most probably corresponds to the agonist binding site (Hibert et al., 1993). In particular, an aspartate residue in TM3 is most probably involved in binding the amino group of the catecholamine side chain (Strader et al., 1988; Hibert et al., 1993). Two serine residues in TM5 have been shown to be hydrogen bond donors to bind the hydroxyl groups of the catechol moiety for the β2- (Strader et al., 1989), α2- (Wang et al., 1991), D2 (Cox et al., 1992; Mansour et al., 1992), and D1 (Tomic et al., 2004) receptors. A phenylalanine in TM6 is highly conserved in all receptors interacting with catecholamine neurotransmitters and can make a stabilizing orthogonal interaction with the aromatic moiety of the ligad. A highly conserved aspartate residue in TM2 has been shown to play a crucial role in β2-adrenergic (Strader et al., 1988; Hibert et al., 1993 Chung et al., 1988), α2-adrenergic (Wang et al., 1991), and D1 (Tomic et al., 1993) and D2 dopaminergic (Neve et al., 1991) receptor activation and to affect agonist binding (Hibert et al., 1993; Tomic et al., 2004; Sidhu et al., 1992). It has been suggested that the interaction between this aspartate and the agonist is allosteric and can be modulated by Na/or H/ (Hibert et al., 1992; Horstman et al., 1990; O’Dowd et al., 1989). A number of cytoplasmic residues, such as the DRY sequence in the second intracellular loop or the alanine residue in the third intracellular loop of the α-adrenoceptor, also play a role in receptor activation (Kjelsberg et al., 1992; Strader et al., 1988).

Benzimidazole piperazinyl (arylpiperazines) affinity

Example 1

Within the scope of the program aimed at the discovery of new dopaminergic ligands, a series of benzimidazoles were synthesized (Soskic and Joksimovic, 1998). The most active compounds were obtained by connecting benzimidazole ring through a flexible spacer with N-arylpiperazines, structure of one such example that produces ligand in such a way is presented in Chart 1. It was observed that the affinity of the obtained ligands for the binding to the D2 dopamine receptor depends on both the structure of benzimidazole and arylpiperazine part of the molecule and the spacer itself (Roglic et al., a, b). In order to obtain a better structure-dopaminergic activity relationship and to evaluate the influence of the spacer in this type of ligands on their binding affinity, various benzimidazolopiperazinyl derivatives have been synthesized, evaluated, and docking analyses of a series of new compounds has been performed (Klocker et al., 2003a, b); therefore, ligand–receptor interaction can be effected on several levels.

Chemistry (Scheme 1)

The derivatives 1–13 and 20–43 (Table 1) were synthesized as outlined in Scheme 1. Compounds with propylene spacer were prepared according to previously described strategy (Roglic et al., 2001a) starting from 4-(3-chloropropyl)-2-nitroaniline (1). Ethylenoxy compounds were obtained starting from 4-(2-chloroethoxy)-2-nitroaniline (17) as a parent compound (Sukalovic et al., 2005). Briefly, 1-(2-chloroethoxy)-4-nitrobenzene (14) was prepared by alkylating 4-nitrophenol with 1, 2-dichloroethane. A simultaneous reduction and acetylation with zinc dust in acetanhydride/acetic acid mixture afforded acetanilide (15). Nitration of 15 in boiling 20 % nitric acid gave o-nitroacetanilide (16) that was further hydrolyzed to o-aniline 17 with boiling HCl. Compound 17 readily alkylates N-phenyl-piperazine in DMF in the presence of K2CO3 and K i at elevated temperature affording 2-nitro-4-[2-(4-arylpiperazin-1-yl)ethoxy]-anilines (18). Further reduction with Ra–Ni/hydrazine afforded o-phenylenediamines 19. Target benzimidazoles (4–8 and 20–24), benzimidazole- 2-thiones (9–13 and 25–32), benzimidazole-2-ones (33–40) and benzotriazoles (41–43) were prepared as described earlier (Soskic and Joksimovic, 1998).

Scheme 1
scheme 1

Synthesis of 5-[3-(4-arylpiperazine-1-yl)propyl]-1H-benzimidazole, 5-[2-(4-arylpiperazine-1-yl)ethoxy]-1H-benzimidazole and their analogs. a Arylpiperazine, K2CO3, DMF. b Ra–Ni, N2H4, EtOH, DCE. c HCOOH, reflux. d CS2, KOH, EtOH. e 1,1′-carbonyldiimidazole and f NaNO2, AcOH. Obtained yields are claimed in parentheses

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Table 1 Chemical structure and binding affinity (Ki) of the examined benzimidazole piperazines and other arylpiperazine ligands for the D1 and D2 dopamine receptors (Sukalovic et al., 2005):
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Example 2: selective dopamine D3 ligands-II

Compound B had high affinity for DA D3 receptors (K i  = 1.5 nM) with much weaker affinity for D2 receptors (K i  = 406 nM) (Svensson et al., 1994). Thus, arylpiperazine analogs of compound 3 were prepared to explore the potency and selectivity of this novel lead for DA D3 receptors (Svensson et al., 2003).

Chemistry (Scheme 2)

The synthesis of compound 3 and analogs is shown in Scheme 2 (Wright et al., 1994) Step (i) could be performed with 2 equivalents of the piperazine instead of using triethylamine to neutralize the HBr formed. However, the piperazines were often in short supply and the use of triethylamine allowed all of the piperazine to be converted to product.

Scheme 2
scheme 2

i Et3N, CH2CI2, reflux, 2 h. ii NaH, DMF, 60 °C, 12 h. iii 1,2-diaminobenzene, NaHSO3, MeOH, reflux, 4 h

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The importance of the phenylpiperazine group for DA D3 binding potency was examined via preparation and testing of the analogs shown in Table 2. Replacement of phenylpiperazine with methylpiperazine (compound 1) greatly reduces DA binding. This result clearly separates the SAR of this series from that of the previously described dimeric series. In that series, dialkylamines were the most active; phenylpiperazine was inactive. As the phenyl was important for DA D3 binding, a Topliss analysis of substituents on the phenyl ring was undertaken (the aryl piperazine starting materials are all known).

Table 2 DA receptor binding for compound B and its analogs (Wright et al., 1995)
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The 4-chlorophenyl analog 2 had weaker DA binding affinity than the phenyl parent B. The Topliss analysis thus suggested that 4-methoxyphenyl analog 3 although had weaker DA D3 binding, but improved D2 binding. In this case, the Topliss analysis proposed the 3-chlorophenyl analog 4. This compound also had weaker affinity for DA D3 receptors. The Topliss analysis, along with several other analogs not reported here concluded that aromatic substitution on the phenylpiperazine group of B will not enhance DA D3 binding.

An unexpected result was seen with 2-substituted phenyl compounds 5–7. These had high affinity for DA D3 receptors, but were disappointingly non-selective due to strong binding to D2 receptors. The 2-pyridylpiperazine and 2-pyrimidylpiperazine analogs 8 and 9 were both reasonably potent at DA D3 receptors but were again less interesting due to moderate affinity for D2 receptors.

From this study, compound 3 showed optimal affinity and selectivity for DA D3 receptors and was evaluated further.

Example 3

During the last decade, the strategy focused on drug design and synthesis of mixed dopaminergic–serotonergic compounds was followed with a possible atypical neuroleptic potential. One of the synthetic approaches was the design and synthesis of heterocyclic arylpiperazines with a specific structure of heteroaryl group that mimics catechol moiety of the dopamine (benzimidazole, substituted benzimidazoles, benztriazoles, and 1,4-dihydroquinoxaline-2,3-diones). Variations of heterocyclic and aryl groups had produced over 100 new compounds that were screened for their affinities at bovine brain D1, D2, and 5-HT1A receptors by in vitro radioligand binding assays (Dukic et al., 1997a, b; Kostic-Rajacic et al., 1998) The six most active compounds (Table 3) were selected for further pharmacological examinations.

Table 3 DA receptor binding affinity and inhibition of 5-HT uptake for compounds 1–6 and clozapine (84)
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All compounds expressed a higher affinity for the 5-HT1A receptors comparing to clozapine for up to one order of magnitude (Table 3). Only two compounds (2 and 3) showed a higher affinity for 5-HT2A than D2 receptors, and this was more prominent for compound 2 (K i of 5.10and 51.3nM at 5-HT2A and D2 receptors, respectively). Compound 2 expressed a comparable affinity to clozapine for the D2, 5-HT2A, and α1-adrenergic receptors, and somewhat lower affinity for the 5-HT2C receptors. It exhibited a poor affinity at the D1 receptor and no binding potency at the 5-HT3 receptor. On the other hand, compound 2 showed a higher affinity for the 5-HT1A receptor than clozapine (K i values of 90.2 and 415 nM, respectively) and a certain potential to inhibit in vitro synaptosomal 5-HT uptake (IC50 = 260 nM). However, the inhibitory strength of compound 2 for 5-HT reuptake, although higher than that of compounds 1 and 3, seems to be fairly low to take into account its possible antidepressive ability by this mechanism, proposed for some new atypical APDs (Meltzer et al., 2003).

Example 4: chemistry

Drug design strategies toward novel dopamine D2 agonists have been based predominately around the endogenous neurotransmitter dopamine (DA, compound 1). (Kaiser and Jain, 1985; Seyfried and Boettcher, 1990; Wikstrom, 1992). Traditional dopamine agonists have a close resemblance to DA (1), most having the ‘3-OH-phenethylamine’ DA pharmacophore (2) or a bioisosteric surrogate, embedded within their molecular structure, e.g., compound 3 (Wikstrom et al., 1989). Studies from various laboratories have resulted in the emergence of a new generation of dopaminergic agents that no longer rely upon the ‘3-OH-phenethylamine’ framework (Mewshaw et al., 1998a, b, c).

In the Scheme 3, the synthesis and structure–activity relationships of various indole, indolone, benzimidazolone, and benzimidazole derivatives (i.e., 5–7 and 9–13) based on the 3-OH-Nl-phenylpiperazine DA D2 template (4) is disclosed. The initial efforts resulted in the identification of several phenolic D2 agonist prototypes (e.g., 4) (Mewshaw et al., 1998a, b, c) that can be used as templates for the design of bioisosteric analogs. As part of a program to discover compounds that could be potentially useful as antipsychotic drugs, embarkment on exploiting these phenolic prototypes by preparing several heterocyclic bioisosteric analogs is done.

Scheme 3
scheme 3

Synthesis and structure-activity relationships of various analogs based on the 3-OH-Nl-phenylpiperazine moiety

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Shown in Tables 4 and 5 are the affinities of the target compounds (i.e., 5–7 and 9–13) for the D2-1ike receptors. The affinities of compounds for the D2 receptors in rat striatal membranes were determined for both the agonist state (high affinity state, D2High) and the antagonist state (low affinity state, D2Low). The D2High state was labeled with [3H] quinpirole (in the absence of GTP and sodium) and the D2Low state was labeled with [3H] spiperone (using ketanserin to exclude 5-HT2 receptor binding) in the presence of GTP The ratio K i (D2Low)/K i (D2High) was used as a preliminary and reliable estimate of the compounds’ intrinsic activity as determined by other assays (Lahti et al., 1992; Wasik et al., 1996). The D2 partial agonist (S)-3-PPP [(K i (D2Low)]/K i (D2High) = 33], was used as a benchmark from which a compound’s estimated intrinsic activity was compared. Affinity for the human cloned receptors was determined using membranes from CHO cells labeled with [3H] spiperone.

Table 4 Affinity of indolones (5 and 6) and benzimidazolone (7) for D2–D4 receptors
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Table 5 Affinities of indoles (9–11) and benzimidazoles (12 and 13) and for D2–D4 receptors
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As shown in Table 4, replacement with the phenol moiety of 4 with the indolone bioisostere (i.e., compound 5) resulted in a tenfold increase in affinity for the D2High receptor and a similar predicted intrinsic activity ratio [(K i (D2Low)/K i (D2High) = 14]. Indolone 5 was observed to have high affinity for all the D2-like receptors and was one of the most potent compounds identified in this study. Though the 5-chloro derivative (6) had unimpressive affinity for the D2High receptor, it was observed to exhibit selectivity for the hD4.4 receptor. Benzimidazolone 7 had similar affinity to its phenol analog (4) with a slight preference for the hD4.4 receptor. Indole, 9 (Table 4) was found to have similar D High2 affinity as its phenol prototype (i.e., compound 4); however, a lower predicted intrinsic activity than 4 was observed [9; (K i (D Low2 )/K i (D High2 ) = 5]. Introduction of either a bromine or chlorine into compound 9 (i.e., 10 and 11) resulted in compounds having selectivity for the hD4.4 receptor. Though benzimidazole 12 had good affinity for the D2-High receptor, introduction of the trifluoromethyl group led to a 19-fold increase in D2-High affinity. In fact, the benzimidazole (13) had similar affinity as its indolone analog (5) for the D2-High receptor, revealing that the indolone and 2-CF3-benzimidazole groups can both serve as surrogate phenol bioisosteres.

Consistent with the predicted low intrinsic activity, in vivo studies showed both 5 and 13 to reduce spontaneous locomotor activity (ED50 = 0.12 mg/kg sc for 13; 5 decreased activity at 0.01, 0.03, 1, and 3 mg/kg sc) and to inhibit apomorphine-induced stereotypy (S) and climbing (C) in mice (ED50s for 13: S = 13.8 mg/kg sc, C = 0.4 mg/kg sc; for 5: S = 1.5 mg/kg sc, C = 2.3 mg/kg sc). Unlike 5, benzimidazole 13 induced catalepsy at 10 mg/kg sc in mice, which correlates with their predicted intrinsic activity ratios (6 vs 14).

Conclusion

It is concluded that among the aryl piperazines, benzimidazolo-piperazines together are effective in binding to the dopamine receptors as well as 5-HT receptors. By varying the substitutions on benzimidazolo-piperazinyl moiety, the compound affinity and preference to various dopamine receptor subtypes are altered and thereby their binding constants are altered. For suitable binding, there is a requirement of a suitable distance between hydroxyl (if present) and a distant nitrogen atom which can be studied through docking analysis. Also, benzimidazolones (indoles and indolones) piperazinyl class of derivatives has been found to possess varying D2 and D4 affinity. Also, the long alkoxy chain separating the benzimidazole and piperazine has given derivatives with increased D3 receptor affinity. There is a wide scope for the synthetic scientists to synthesize more and more compounds with different substitutions using benzimidazole and piperazine as a basic moiety. Thus, investigation and probing of the boundaries of the D2 agonist pharmacophoric criteria is being continued by identifying potential antipsychotic agents that belong to this new generation of dopaminergic agents which in turn could possess optimum antipsychotic activity.